In the field of color television, television cameras utilize image tubes such as for example vidicons, Plumbicons, Saticons, etc., as pickup tubes. Most successful of the tubes is the Plumbicon. A typical Plumbicon image tube utilizes a cathode connected to a -45 volt supply and a target on the face plate thereof, whereby during normal operation, light falling on the target causes the side of the target which is facing the cathode to charge positively towards ground potential. When the scanning beam reads off this part of the target, it attempts to discharge the target back towards the cathode potential, i.e., -45 volts. In the case of normal light levels falling on the target, the beam current available from the cathode is sufficient to discharge the target back down to -45 volts in one or two passes of the scanning beam. Thereafter, during the next field, the incoming light will charge the target back up toward ground potential again, whereupon the scanning beam will reduce the charge back towards -45 again on the next pass, etc.
However, when the incoming light falling on the target is radically increased, i.e., a condition known as a "high-light," the scanning beam has insufficient current to discharge the target back down to -45 volts in one pass of the scanning beam. If the camera is panned across a highlight, e.g., a very bright object, an image of the highlight, slowly decaying with time, will appear where the highlight was previously. Thus, several passes of the scanning beam are required to fully discharge the highlight area of the target back down to cathode potential. Simultaneously, however, signal current is being produced from the Plumbicon corresponding to the conventional video image. The result is a smearing of the image in the areas of the highlight, an effect known as "comet-tailing."
A related problem exists when there is insufficient beam current to fully discharge the target in one pass of the scanning beam, which is called "blooming." This occurs when the target area on which the highlight falls develops the large positive charge, whereby the electron beam is pulled towards the area far more rapidly than it should be by the attraction of the positively charged area on the negative electrons. Likewise, as the electron beam leaves the area, it is pulled back a little and tends to leave the area more slowly than it should. The effect of this condition is a time deformation of the image. For example, if the camera is imaging a small white square, it will actually appear as a larger white square on the television monitor. Blooming is also aggravated by the fact that the highly positively charged area of the target tends to bleed out charge in all directions away from the highlight due to leakage effects of the layer. This causes additional blooming around the outer regions of the highly positive area.
There are a number of potential solutions to the above problems of comet-tailing and blooming. In a first solution, the beam current from the cathode may be increased during the forward active picture scan to handle significant highlights. However, there are two disadvantages to this scheme. One disadvantage is that the resolution of the tube is greatly impaired by virtue of the fact that the beam profile becomes larger and is therefore incapable of resolving fine detail in the image. Secondly, the life of the pickup tube is greatly impaired, i.e., is reduced by a factor of two or three to one over what the expected life would be from a tube operated at normal beam current. For this reason, a compromise is made in selecting the normal value of the tube beam current which is small enough to allow adequate resolution to be obtained, yet large enough to allow reasonable highlight handling without greatly impairing the life span of the tube.
In an attempt to improve the highlight handling characteristics of the image tubes, another technique was developed involving a specially designed tube, commonly termed an "anti-comet-tail (ACT)" tube. The special tube includes an additional auxiliary grid of special configuration, and a cathode structure that allows a very large beam current to be generated in order to prevent the highlight effects. Relatively complex circuitry also is needed to support the tube and to drive the various control grids in order to handle the highlights. More particularly, during the normal horizontal retrace, i.e., the beam flyback scan, instead of turning the beam off, the beam is turned up to an extremely high level, i.e., hundreds of times normal beam current. To prevent damage to the tube by this excessively high current, the beam is heavily defocussed. This is performed by the auxiliary grid electrode to which is applied a suitable drive pulse. The result is that the flood of electrons covers many scanning lines and is less likely to damage the tube. The flood of electrons in turn discharges any areas of the target that are not normally discharged by the much lower value of beam current used during the forward scan.
To prevent the high current electron beam from reading out normal video images during retrace scans, the cathode potential of the tube was increased on the order of 7 volts above its normal -45 volt value, e.g., to -38 volts. Thus, the electron beam lands on any area of the target that is charged more positively than approximately -38 volts. It follows that by turning up the cathode voltage during the flyback scan, the beam electrons only land on parts of the target that have been affected by extreme highlights. It is to be understood that the values given above are by way of example only and correspond to average conditions where normal video signals give rise to charges on the target of the order of 6 or 7 volts peak-to-peak, such that the target varies from -45 to about -38 volts over a period of one field in response to a peak normal 100% video signal.
Various disadvantages exist in the special ACT tube and associated circuitry. For example, the resolution of the tube is inferior to that of a traditional Plumbicon tube. Secondly, the lifetime of the special tube is of the order of 30% less than the traditional Plumbicon tube. The latter condition is caused by the generation of X-rays in the vicinity of the target by the high velocity of the electron beam landing on the metallic grids of the electrodes of the special tube. Thus the use of ACT tubes for preventing the negative effects of highlights has never gained popularity.
In a further attempt to overcome the negative effects of highlights, a new approach for handling the beam during the forward, active picture scan was developed. In this scheme, the beam current in the tube is kept at a safe static value, except when a highlight appears on the picture. Then, during the forward scan, the beam current is suddenly increased to a value sufficient to discharge this highlight. As soon as the scanning beam passes the highlight area, the beam current again is decreased to its normal operating value. Thus, for example, when a highlight appears in the image, the beam current is normal until such time as the scanning beam reaches the highlight area. The video signal generated by the preamplifier coupled to the target will increase due to the highlight, and the increased signal is fed back to the grid of the tube. This in turn causes the beam current to increase to some large predetermined value, which allows the tube to successfully read out the highlight during the forward scan. As soon as the scanning beam passes the highlight area, the increased video signal vanishes and the grid voltage is reduced to normal value.
The latter beam control system suffers various disadvantages, the most notable of which is the fact systems like this have a very large tendency to oscillate. The reasons for oscillations are complicated, but suffice it to say that when the beam current is suddenly increased, the scanning beam is deflected from its normal path and may, for example, move up or down one or two lines of the picture. It follows that on the next pass of the scanning beam, the beam may read part of the image that it previously read, thereby generating a black line rather than a white line. The resulting pattern of dark and white lines looks very much like oscillation. Such oscillation may be minimized or eliminated by very careful and complex circuit design. But because of development trends in the portability, automation, etc., of broadcast quality television cameras, simplicity of design is a must if a competitive broadcast camera is to be provided.
Further disadvantages of the latter scheme arise due to the finite response time of the system, wherein the scanning beam must detect the highlight to cause the beam current to increase. Thus, very small speculars of highlight in the picture are passed by the electron beam before the system can respond to increase the beam current. Therefore, for example, if a small highlight such as generated by a piece of tinsel, a diamond earring, etc., is scanned, the system does not turn itself on in time to prevent the negative highlight effect. A further disadvantage of having a finite response time is that the leading edge of the highlight is not discharged properly in some cases. Therefore, if the camera is panned across a highlight, the leading edge of the highlight can actually bleed off and appear as a very short comet-tail. The above disadvantages can usually be overcome by increasing the response speed of the system. However, the faster the system is, the greater is the tendency for it to oscillate. And, as discussed above, such a beam control system, with feedback, of necessity includes relatively complex circuitry.
The disadvantage of having oscillation is compounded by the fact that different tubes of the same family, and/or of different sizes, have different characteristics. Thus, while the scheme may work on one particular tube, on another type of tube it may cause oscillation. In specific systems where the tube works well, frequent and critical adjustments of the cameras still are necessary in order to maintain the prevention of oscillation.
The present invention overcomes the shortcomings of the prior art by providing an automatic beam control system which provides the advantages, and none of the disadvantages, of all the above-mentioned beam control systems. In particular, the invention combines the advantages of the special ACT tube system which operates during the retrace (flyback) scan with the advantage of, automatic beam handling system with standard Plumbicon, Saticon, etc., tubes, which operates during the forward, active picture scan. The invention system utilizes the fact that standard pickup tubes, and particularly diode-gun Plumbicons, have considerable increased beam reserve, whereby the system circumvents highlight effects using a standard pickup tube. The system therefore does not require a special ACT tube and associated drive circuitry, or the complex automatic beam control system circuitry, but is, in fact, extremely simple to implement with a standard pickup tube.
More particularly, the invention system increases the beam current to the maximum possible value which can be handled by the tube during the horizontal retrace, while raising the cathode voltage a select amount to prevent readout of normal video information during the active picture scan. The latter feature is further optimized by modifying the scanning beam path during the retrace scan to positively insure that the image is scanned by the retrace scan beam prior to being scanned by the forward scan beam.
Accordingly, it is an object of the present invention to provide a very simple, automatic beam control system utilizing a standard Plumbicon, Saticon, vidicon, etc., image tube.
It is another object to provide an improved automatic beam control system which discharges excessive highlights during the rapid retrace scan period.
Another object is to provide an improved highlight supression system for a television pickup tube utilizing a modified retrace beam path.
Still another object is to provide a highlight suppression technique for television pickup tubes which insures that a given portion of the tube target always is scanned by the retrace scan beam prior to being scanned by the forward active picture scan beam.
It is still a further object to provide highlight suppression in a television camera via an extremely simple circuit which is totally free of instabilities inherent in prior art highlight suppression systems.